Modern telecommunications networks are comprised of input and output devices, e.g., telephone sets, local central offices, and one or more telephone switches that are used for processing voice signals in the network. Voice signals can be characterized as containing two regions, including the bass region and the treble region. The bass region is typically considered to be that part of the voice signal below 300 hertz (Hz), and the treble portion is that part of the signal above 300 Hz. Voice signals may be attenuated in the bass band region by one or more of the elements in a telecommunication network.
The Electronic Industry Association (EIA) standard RS-470, published January 1981, recommends that the input voice signal be attenuated below approximately 300 Hz by the codec in the input telephone station. This attenuation of the amplitude of the bass portion of the input voice signal is recommended because background noises in a telecommunication network lie in the bass region. By decreasing the amplitude of the bass portion of the input signal, the background noises of the network are also diminished.
Additionally, central offices of a telecommunication network may also attenuate the bass band region of a voice signal. Within a central office may be located a channel bank that converts the analog input voice signal to its digital equivalent. The digital voice signal is coupled to the receiving telephone set by a digital telephone switch or switches. Before the signal is provided to the receiving telephone set, it is converted back to analog format at another central office located between the last switch and the receiving telephone set. The channel banks may attenuate the bass portion of the input voice signal during the analog to digital conversion process.
Some networks, therefore, attenuate the bass region of the input voice signal twice; in the input telephone set and in the central office. Attenuation of the bass region of the input signal results in a voice signal at the receiving telephone set that is not a true representation of the speaker's voice. Techniques have thus been proposed to compensate for the loss of bass in a telephone speaker's voice.
One prior approach for providing an enhanced voice signal in a telecommunications network utilizes a fixed gain technique. In the fixed gain approach, the bass portion of the voice signal is amplified while the signal is in the telecommunications network and before it is supplied to the receiving telephone set. This approach compensates for attenuation of the input signal with a fixed gain at some point within the network. This approach also amplifies the previously noted network background noises within the bass band region.
Moreover, if the input voice signal is a loud signal, i.e., the speaker is speaking at a high decibel (dB) level, the fixed gain enhancement approach will further amplify the high decibel signal, thus resulting in a signal at the receiving telephone set that can be uncomfortable to listen to. Alternatively, applying a fixed gain to a high decibel input signal can result in over-driving/saturating different network elements, making the signal less clear than it would have been if the fixed gain had not been applied.
An additional problem associated with the fixed gain technique for voice enhancement occurs when data is transmitted over the telecommunications network in the voice-band. This is becoming a more frequent occurrence for telecommunications systems as the use of facsimile machines and modems coupling computers continues to grow. A modem or facsimile machine transmits voice-band data at a high amplitude and at a high-frequency, e.g., 2700 Hz. Therefore, should the fixed gain technique be applied to a voice-band data signal, it will be unnecessarily amplified, thus resulting in a voice-band data signal that is difficult to use on the receiving end.
Detectors for sensing the transmission of voice-band data have been employed to solve the problems associated with voice-band data transmissions. These detectors are remote to the fixed gain enhancement circuitry, requiring an external control link to the enhancement circuitry for disabling the circuitry. This ensures that the voice-band data is not amplified.
Another problem associated with previously developed voice enhancement systems occurs when an input voice signal traveling in a telecommunications network encounters or must pass through multiple network elements (tandem network) that include fixed gain voice enhancement circuitry. Current fixed gain voice enhancement systems cannot detect when a input voice signal has already been adjusted by the fixed gain technique. Therefore, a voice signal amplified in a first element of a tandem network may be subsequently again amplified by the second element in the network. This additional amplification can result in the saturation of the voice signal, or at a minimum, make the signal uncomfortable to listen to on the receiving telephone set. Also, multiple enhancements to a voice signal can result in oscillation of the voice signal in the tandem network.
One prior approach for detecting whether a signal has been previously enhanced (tandem detection) involves the generation and detection of a sub-audible tone, typically on the order of 20 Hz as an indication of whether the voice signal has been enhanced. Such a tone passes freely over the central digital network where normally there is no frequency selective filtering. When the voice signal with the sub-audible tone goes off the digital network, however, and is converted to analog before being delivered to the subscriber, the tone is stripped off by a codec and transformer filtering. It is then possible for the subscriber to create the tandemed situation by conferencing back onto the network without the sub-audible tone passing between the tandemed networks. Without the sub-audible tone, this previously enhanced signal may be enhanced an additional time in the network, which may result in an unsatisfactory signal as previously described.